Cathode ray tubes are used extensively for converting electrical signals into a visible image on the face of the tube. They possess the advantage of very high writing speeds which permit the representation of real time information. Since the display itself is relatively small, viewing must be done close up to the tube. Thus, the audience to which the information can be displayed at any given time is limited. Viewing by a large group of people is not possible, particularly in an area where a reasonable amount of ambient illumination must be maintained.
Past attempts at providing a bright display which is at least several feet high have met with varying degrees of success. One promising approach, shown in U.S. Pat. No. 4,127,322, provides a light projection system which employs cathode ray tubes in association with liquid crystal light valves. The light emitted by a bright source of light is controlled and is ultimately projected onto a viewing screen. Each cathode ray tube is used in association with an AC driven, field effect mode, birefringent liquid crystal light valve, wherein the polarization of the projection light from the light source is modulated in response to the low level light image on the face of the cathode ray tube. By using one cathode ray tube and an associated light valve for each primary color, a full color, real time image is produced on a large projection screen, suitable for group viewing.
The detail with which an image is portrayed to the human eye is determined by the "resolution" capability of the image reproducing system, i.e. by the number of recognizable basic picture elements that can be reproduced. The detail perceived from the image is determined in terms of both vertical and horizontal resolution. In a CRT, the beam deflection means typically causes the electron beam of the tube to sweep periodically through a raster pattern. The beam intensity is controlled concurrently with the sweep, so as to control the intensity of the CRT scanning spot responsible for the visible trace of the image on the face of the tube. Horizontal resolution is a function of the number of picture elements that are generated per unit of horizontal distance swept by the beam, as well as of the shape of the scanning spot itself which is controlled through beam focusing means. Vertical resolution depends on the number of horizontal sweep lines of the raster pattern per unit of vertical distance. To increase the resolution of a CRT which has a sharply focused beam, the number of picture elements, as well as the number of horizontal lines, must be increased per unit of horizontal and vertical distance respectively.
It is sometimes advantageous to display one or more portions of the projected image with greater resolution than the surrounding background. This may be done for the purpose of studying the image portion in question and/or to differentiate it from its background. Such viewing of the image is particularly desirable in certain radar applications, such as radar simulation, where one or more selected targets in the sector covered by the radar beam at any given movement may be of particular interest to the viewer. As used herein, the term "target" is intended to indicate that portion of the total image on the viewing screen, which is singled out for detailed display against its surrounding background.
In one embodiment of the electro-optical light projection system disclosed in the aforesaid patent, an additional cathode ray tube and an associated liquid crystal light valve are used to provide a monochromatic, black and white overlay image on the full color background image displayed on the projection screen. It has been proposed to use such an arrangement to display a target in high resolution detail against its low resolution background. To do so, a monochromatic mini-raster can be used, which has a greater number of picture elements and horizontal lines respectively, per unit length than the equivalent parameters of the base raster that forms the background image. Further, since the target occupies only a small portion of the total displayed image, the number of horizontal lines that are used for the mini-raster, as well as its sweep angle on the face of the tube, are both small with respect to the corresponding base raster dimensions.
The arrangement described has a number of disadvantages which limit its utility to a narrow range of situations. One problem arises from false target cues produced by the unnatural color contrasts between the monochromatic mini-raster overlay on the full color background, which may portray blue sky, green grass, etc. Such cues are likely to confuse rather than educate the observer about the target and its background. Another problem arises from the inability to distinguish with clarity any but the brightest targets from their background. A further drawback is the requirement for an additional cathode ray tube and liquid crystal light valve to produce the overlay. The added equipment not only raises the cost of what is already an expensive system, but it materially increases its complexity. This is due to the added timing and image positioning requirements which add to the difficulty of maintaining the system in proper working order.